A battery is an electrochemical device that converts the energy released in a chemical reaction directly into electrical energy. In a battery, the reactants are stored close together within the battery itself, whereas in a fuel cell the reactants are stored externally. The attractiveness of batteries as an efficient source of power is that the conversion of chemical energy to electrical energy is potentially 100% efficient, although the loss due to internal resistance is a limiting factor. This potential efficiency is considerably greater than the conversion of thermal energy to mechanical energy as used in internal combustion engines, which always results in heat transfer losses. Moreover, the additional disadvantages of contaminants emitted into the atmosphere as byproducts of incomplete combustion and the dwindling availability of fuel supplies have intensified research into batteries as an alternative source of energy.
Thin-film battery technology is seen as having several advantages over conventional battery technology in that battery cell components can be prepared as thin, e.g. 1 micron, sheets built up in layers using techniques common to the electronics industry. The area of the sheets can be varied from sizes achievable with present lithographic techniques to a few square meters, providing a wide range in battery capacity. Deposition of thin films places the anode close to the cathode resulting in high current density, high cell efficiency and a great reduction in the amount of reactants used. This is because the transport of ions is easier and faster in thin film layers since the distance the ions must move is lessened.
A thin film battery typically comprises a cathode current collector deposited on a substrate and upon the cathode current collector is deposited a cathode. An electrolyte is deposited onto the cathode and an anode and anode current collector are subsequently and sequentially deposited on the electrolyte. Alternatively, the layer depositions can be reversed. The thin films are typically formed by standard thin film fabrication processes, including physical and chemical vapor deposition methods, sputtering and electroplating.
An all-solid-state high-voltage lithium or lithium-ion secondary battery has potentially superior properties compared to conventional liquid/gel based batteries. All-solid-state lithium batteries are leak proof, exhibit high safety performance, are mechanically robust and can be used over a wide range of temperature. Solid-state batteries are considered very safe as they involve no liquid or organic materials. Such batteries can have a wide temperature range of operation because of the materials involved. They can also exhibit high power as well as high capacity, combined with a low self-discharge rate and without electrolyte leakage. Solid-state inorganic electrolytes have high decomposition voltages up to about 6V against lithium metal and can be, accordingly, employed together with high-voltage lithium cathode materials. For example, lithium phosphorous oxy-nitrides Li3-xPO4-yNy (LiPON) and garnet-like structure Li6BaLa2Ta2O12 exhibit high ionic conductivity and high electrochemical stability at room temperature.
In general, solid lithium electrolytes have a transference number of the lithium ions of 1. This is in contrast to common liquid and polymeric electrolytes, where both cations and anions are mobile. Often, transference numbers of the lithium ions are much smaller than those of the anions. This high mobility of ions other than lithium may lead more readily to the formation of solid electrolyte interfacial (SEI) layers, which may cause deterioration and limit the life-time of the batteries. The negligible mobility of ions other than the electroactive ones in the solid state may provide a superior chemical stability. Depending on the lithium activity, however, electronic species may become important at high and low lithium activities. It should be also mentioned that all solid-state batteries will have low gravimetric energy density due to the low weight of the packaging materials compared to conventional rocking-chair batteries.
One problem encountered in the use of solid-state thin film batteries is the fracturing of the films due to stresses caused by expansion and shrinkage during the charge-discharge cycles using standard materials in the thin film. Useful would be a thin film solid-state battery that incorporates materials that do not exhibit significant stresses during the charge-discharge cycles as well as battery geometries that mitigate such effects.
Therefore, high lithium-ion conductivity thin film electrolytes for solid-state lithium batteries are desired for reduced package size, increased safety, and enhanced power and energy density. There have been extensive efforts to develop solid-state lithium-ion conductors appropriate for integration as electrolytes, including several candidate materials possessing room temperature ionic conductivities of up to 10−3 S cm−1. See Y. Inaguma et al., Solid State Commun. 86, 689 (1993). In spite of these high reported conductivity values, many of these compositions suffer from issues that prevent their use in lithium-ion cells. For example, (Li,La)TiO3 and Li1.3Al0.3Ti1.7(PO4)3 both display high ionic conductivities but are unstable in contact with lithium metal due to facile Ti4+ reduction. See V. Thangadurai et al., J. Am. Ceram. Soc. 86, 437 (2003). Owing to its stability in contact with lithium, ease of manufacture, and outstanding cyclability, the most widely used solid electrolyte films are based on amorphous LiPON compositions. See N. J. Dudney, Electrochem. Soc. Interface 17, 44 (2008). In this system, modest room temperature ionic conductivities of ca. 10−6 S cm−1 can be achieved through processing conditions compatible with many packaging embodiments. See X. H. Yu et al., J. Electrochem. Soc. 144, 524 (1997). In spite of the success and widespread use of LiPON in thin film solid-state batteries, there is still a strong desire to develop lithium-stable, higher-conductivity electrolytes with similar processing advantages, but with the potential to enable energy storage devices with greater power.
High lithium-ion conductivities have been observed in the A-site deficient perovskite (general formula ABO3) La1/3-xLi3xTaO3 (referred to hereinafter as LLTO), which is iso-structural with the well-known fast lithium-ion conducting La2/3-xLi3xTiO3. Lanthanum lithium tantalate perovskites offer the possibility of room temperature ionic conductivities up to 10−4 S cm−1 with a valence stable B-site cation, tantalum. See K. Mizumoto, and S. Hayashi, J. Ceram. Soc. Jpn. 105, 713 (1997); K. Mizumoto and S. Hayashi, J. Ceram. Soc. Jpn. 106, 369 (1998); K. Mizumoto and S. Hayashi, Solid State Ionics 116, 263 (1993); and K. Mizumoto and S. Hayashi, Solid State Ionics 127, 241 (2000). Thin films of La1/3-xLi3xTaO3 (x=0.06) have been prepared via a sol-gel method on SiO2 and single-crystalline SrTiO3 substrates, but required processing to temperatures in excess of 900° C. and oxidizing atmospheres to realize high conductivities. See S. Arakawa et al., J. Cryst. Growth 231, 290 (2001). While successful in achieving the appropriate phase and high ionic conductivities, the use of these insulating, refractory, and expensive substrates limits commercial viability and the potential range of applications. Additionally, the necessary high-temperatures and oxidizing atmospheres used during the processing steps to crystallize La1/3-xLixTaO3 are generally incompatible with standard solid-state cell embodiments, which utilize lithium metal anodes, copper current collectors, and nano-crystalline cathodes. A method to circumvent processing temperature and atmosphere limitations has been developed in ‘lithium-free’ cells whereby heterostructures of an electrolyte processed directly on a cathode are prepared. See B. J. Neudecker et al., J. Electrochem. Soc. 147, 517 (2000). In these cells the first charge cycle can be used to plate lithium metal at the electrolyte-anode current collector interface and thereby avoid the tight processing controls necessary to deposit and handle metallic lithium. By reversing the battery stacking sequence and depositing the electrolyte first and capping with a cathode a hermetically sealed cell can be formed. See S. H. Lee et al., Electrochem. Solid State Lett. 6, A275
(2003). Additionally, this preparation sequence provides a means to prepare solid electrolytes requiring high processing temperatures while avoiding potential degradation of the cathode material.
While many solid-state battery technologies utilize rigid substrates, the use of a thin and flexible substrate is advantageous for reel-to-reel manufacturing, flexible electronics, and for applications requiring low profiles. Solid-state lithium-ion batteries have previously been prepared on flexible substrates, including 80 μm thick aluminum foil, using sputter-deposited LiPON as an electrolyte. See S. H. Lee et al., Electrochem. Solid State Lett. 2, 425 (1999). This cell utilized low temperature physical vapor deposited components to maintain nano-crystalline anodes and cathodes and to prevent deterioration of the aluminum foil substrate when exposed to oxidizing atmospheres at high temperatures. Thus, this substrate material limits the allowable thermal budget and precludes the use of most crystalline oxide electrolytes.
Therefore, a need remains for a method to fabricate lanthanium-lithium-based electrolytes on flexible metal substrates.